Lithography exposure systems for the EUV wavelength range (from about 5 nm to about 20 nm) generally comprise an EUV light source, an illumination system for homogeneously illuminating a pattern arranged on a mask with light from the EUV source, and a projection system for imaging the pattern onto a photosensitive substrate (wafer). In the present application, the term “light” designates electromagnetic radiation at wavelengths which are not restricted to the visible domain, i.e. the term “light” will be used also for radiation in the EUV or VUV wavelength range.
During the exposure process, a contamination layer containing mainly carbon grows on the optical surfaces of the optical elements of the EUV-lithography system. The formation of the contamination layer is triggered by radiation-induced cracking of hydrocarbon molecules, the presence of which cannot be avoided even though the compartments of the EUV-lithography system are operated under vacuum conditions. The contaminating material, in particular carbon, can be cleaned away by bringing the contamination layer on the optical surfaces into contact with a cleaning gas, such as atomic hydrogen. Currently, it is foreseen to equip each EUV-reflective element of a EUV lithography system with one hydrogen radical generator (HRG), such that cleaning can be performed in-situ, i.e. without removal of the optical elements from the EUV-lithography system. In the present application, the term “atomic hydrogen” is used for all types of activated hydrogen (H2), i.e. not only designating hydrogen in the form of hydrogen radicals H·, but also hydrogen ions such as H+ or H2+ or hydrogen H* in an excited (electron) state.
In general, the amount of material removed from the contamination layer by the cleaning gas cannot be precisely determined. Consequently, the cleaning time during which the cleaning gas should be brought into contact with the contamination layer is only approximately known. In case that the cleaning time is too short, part of the contamination layer will not be removed from the optical surface, causing an unwanted reflection loss even after the cleaning. Therefore, for ensuring that the entire contamination layer is removed by the cleaning, a cleaning time may be chosen which is too long (so-called “overcleaning”) so that, especially close to the end of the cleaning process, part of the cleaning gas may come into contact with the optical surface. As the optical surface in general also reacts with the cleaning gas, an irreversible contamination is caused on the optical surface in a low amount per cleaning cycle. As only 1% reflection loss due to irreversible contamination is permitted over the lifetime of a EUV-reflective element, the lifetime of the optics of a EUV-lithography system is determined by the mean time between successive cleaning processes multiplied by the number of allowed cleaning cycles.
In the publication US 2003/0051739 A1, a device for removing carbon contaminations from an optical surface of a mirror element in a EUV lithography system is disclosed. In one example, the device comprises two cleaning gas generators, each for generating a jet of cleaning gas which is directed to the optical surface. In another example, a single cleaning gas generator of cylindrical shape is used which is situated around the perimeter of the mirror. The cleaning gas is generated by activation of a supply gas using accelerated electrons which are produced by thermoemission from a heated filament.
WO 2004/104707 A2 discloses a method for in-situ cleaning of an optical surface of an optical element for EUV or soft X-ray radiation which is arranged in a vacuum vessel, the optical surface being contaminated with an inorganic substance introduced by a radiation source. In the method, at least one reagent which is substantially translucent or transparent to the radiation (such as molecular hydrogen) is added through a supply system of a vacuum vessel, depending on the prevailing conditions. The reagent chemically reacts with the contaminants to remove them from the optical surface. The reagent may be activated by irradiation with an activation light source and may be generated in a pulsed manner. The method may also be electronically controlled, e.g. by taking the thickness of the contamination layer into account.